Report Number: CS-TR-96-1577
Institution: Stanford University, Department of Computer Science
Title: A More Aggressive Use Of Views To Extract Information
Author: Huyn, Nam
Date: December 1996
Abstract: Much recent work has focussed on using views to evaluate
queries. More specifically, queries are rewritten to refer to
views instead of the base relations over which the queries
were originally written. The motivation is that the views
represent the only ways in which some information source may
be accessed. Another use of views that has been overlooked
becomes important especially when no equivalent rewriting of
a query in terms of views is possible: even though we cannot
use the views to get all the answers to the query, we can
still use them to deduce as many answers as possible. In many
global information applications, the notion of equivalence
used is often too restrictive. We propose a notion of
pseudo-equivalence that allows more queries to be rewritten
usefully: we show that if a query has an equivalent
rewriting, the query also has a pseudo-equivalent rewriting.
The converse is not true in general. In particular, when the
views are conjunctive, we show that all Datalog queries over
the source do have a pseudo-equivalent Datalog query over the
views. We reduce the problem of finding pseudo-equivalent
queries to that of rewriting Horn queries with Skolem
functions as Datalog queries. We present an algorithm for the
class of term-bounded Horn queries. We discuss extending the
problem to larger classes of Horn queries, other non-Horn
queries that result from ``inverting'' Datalog views and
adding functional dependencies. The theory and methods
developed in our work have important uses in query mediation
between heterogeneous sources, automatic join discovery and
view updates.
http://i.stanford.edu/pub/cstr/reports/cs/tr/96/1577/CS-TR-96-1577.pdf